1. Field of the Invention
The present invention relates to a radiographic intensifying screen set (hereinafter referred to as intensifying screen set) comprising a pair of a front intensifying screen and a rear intensifying screen, for radiographing.
2. Discussion of Rearground
With respect to a radiographic intensifying screen (hereinafter referred to as intensifying screen), in order to improve sensitivity or sharpness, various improvements have heretofore been made, such as improving light-scattering characteristics of a support or a protective layer to be used for it, making phosphor layers have a multi-layer structure, or coloring the phosphor layers by e.g. a dye. However, it has been desired to develop an intensifying screen having more improved sensitivity and image quality, in the market. Further, with respect to the improvements in characteristics of an intensifying screen, although numbers of proposals have been made with respect to improvements in mainly characteristics of the intensifying screen alone, only a few proposals have been made with respect to constitution of the intensifying screen as an intensifying screen set.
It is an object of the present invention to provide an intensifying screen set which is excellent in both sensitivity and sharpness, and which presents a radiograph having an excellent image quality.
The present inventors have conducted extensive studies on correlation between an intensifying screen set comprising intensifying screens having various constitutions, and image quality of a radiograph which will be obtainable by using the intensifying screen set, and they have found that the above-mentioned objects can be achieved when each of phosphor layers for front and rear intensifying screens have a multi-layer structure, a fluorescent dye or a fluorescent pigment is contained in the phosphor layers, and particle size distribution of phosphors in the phosphor layers and light-reflection characteristics of the supports are specified.
The present invention provides:
(1) a radiographic intensifying screen set comprising a pair of a front intensifying screen and a rear intensifying screen, each comprising a support and a plurality of phosphor layers each having a binder resin and a phosphor dispersed therein, provided on he support, wherein at least some of the phosphor layers of the respective front intensifying screen and rear intensifying screen contain a fluorescent dye or a fluorescent pigment which absorbs some of emitted lights from the phosphors and emits lights having other wavelengths, the support for the front intensifying screen is a light-reflective support, and the support for the rear intensifying screen is a light-absorptive support,
(2) the radiographic intensifying screen set according to the above-mentioned (1), wherein the phosphors in the phosphor layers are aligned so that the average particle size decreases from the surface side (the side from which the emitted lights are taken out) toward the support side, and
(3) the radiographic intensifying screen set according to the above-mentioned (1) or (2), wherein a light-scattering protective layer having a transmission of at least 40% and a haze ratio of at least 20%, is provided on the surface of each of the front intensifying screen and the rear intensifying screen.
Now, the present invention will be explained in further detail.
The intensifying screen set of the present invention can be produced in the same manner as a conventional intensifying screen set, except that a fluorescent dye or a fluorescent pigment (hereinafter such fluorescent dye and pigment will generically be referred to simply as xe2x80x9cfluorescent dyexe2x80x9d) is added to at least some of the phosphor layers of the respective front intensifying screen and rear intensifying screen constituting the intensifying screen set, the phosphor particles in the phosphor layers are specifically aligned, and supports having specific light-reflection characteristics are employed.
Namely, both front intensifying screen and the rear intensifying screen constituting the intensifying screen set, are produced in such a manner that a predetermined amount of phosphor and fluorescent dye are mixed with a binder such as nitrocellulose, an organic solvent is further added thereto to prepare a phosphor coating solution having a suitable viscosity, the coating solution is coated on each of the supports by e.g. a knife coater or a roll coater, followed by drying to form a phosphor layer, and a protective layer is further formed on this phosphor layer, as the case requires, to obtain a front intensifying screen and a rear intensifying screen.
Here, the front and rear intensifying screens constituting the intensifying screen set of the present invention, do not have a phosphor layer of single-layer structure which is formed by one kind of phosphor coating solution having phosphors having a single particle size distribution suspended therein, but have a plurality of phosphor layers on the respective supports, which are formed by using at least two kinds of phosphor coating solutions having different particle size distributions, coating one phosphor coating solution on the respective supports, and after drying or in a non-dried state, successively coating another phosphor coating solution thereon. In such a case, when a plurality of phosphor layers are formed by successively coating phosphor coating solution from one containing a phosphor with a smaller average particle size on the support, and when the phosphor particles are aligned so that the particle size is smaller at the support side than at the surface side (the side from which the emitted lights are taken out), the layer of the phosphor having a smaller particle size than the surface side, aligned at the support side, functions as a close reflective layer of lights, and suppress scattering of emitted lights from the phosphors. Accordingly, as compared with a case where phosphors having a uniform particle size distribution are uniformly aligned on the entire phosphor layer, an effect to improve the sharpness, in addition to an effect to improve sensitivity by the light-reflective support, can be obtained in the case of the light-reflective support, and in the case of the light-absorptive support, an effect to improve sensitivity, in addition to an effect to improve sharpness, can be obtained. Accordingly, effects to improve sensitivity and sharpness can be obtained for both supports.
In order to improve dispersibility of phosphors in the respective coating solutions, it is preferred to carry out a dispersion treatment by e.g. a ball mill to the phosphor coating solutions, after the phosphor coating solutions to form the respective phosphor layers are prepared and before coated, as the packing density of the phosphors in the phosphor layers after coating will be high, and the uniformity of the coated phosphor layer will be excellent.
In the intensifying screen set of the present invention, the fluorescent dye to be contained in the phosphor layer, may be contained in the phosphor layer of at least one of front and rear intensifying screens. With respect to the content (percentage by weight of the added fluorescent dye to the weight of the phosphors in the phosphor layers) in the phosphor layers of both front and rear intensifying screens, if it is small, absorption of the emitted lights from the phosphors will be inadequate, and if it is high, the absorption amount of the emitted lights from the phosphors will be higher than the emission amount from the dye, whereby sensitivity will be decrease. Accordingly, the content of the fluorescent dye to be added to the phosphor layer is preferably at the level of from 0.001 to 0.05%, more preferably at the level of from 0.002 to 0.02%, to the phosphors, depending upon the type of the fluorescent dye to be added, as the absorption amount of the emitted lights from the phosphors and the emission from the fluorescent dye will be well balanced.
As mentioned above, when a fluorescent dye is added to the phosphor layer of at least one of front intensifying screen and the rear intensifying screen, some of the emitted lights from the phosphors are absorbed by the fluorescent dye contained in the phosphor layer until the emitted lights reach the surface of the intensifying screen, and converted to fluorescent lights having longer wavelengths. By adjusting the emission wavelength after conversion to the spectral sensitivity of a film emulsion, the sensitivity will improve, and the absorption rate of the emitted lights at the film emulsion layer will be high, whereby the crossover effect due to transmission of lights through film base will decrease, and sharpness will improve.
The fluorescent dye to be used in the present invention is a substance which absorbs some of the emitted lights from the phosphors in the phosphor layers, and emits visible lights. For example, when Gd2O2S:Tb phosphor is used as the phosphor, this phosphor emits lights having main peak wavelengths of emission spectrum in the vicinity of 545 nm, and the fluorescent dye is one which absorbs emitted lights except lights having wavelengths in the vicinity of 545 nm, and emits lights having wavelengths in the vicinity of 545 nm. The emission peak wavelength of the fluorescent dye is set to the spectral sensitivity of silver halide photosensitive materials. Specifically, a fluorescent dye or a fluorescent pigment of an organic or inorganic compound which absorbs lights having wavelengths of at most 500 nm, and which has an emission peak within a wavelength range of from 450 to 600 nm, is preferred. The emission peak is more preferably within a range of from 490 to 600 nm, particularly preferably within a range of from 500 to 570 nm.
Such a fluorescent dye has an emission quantum yield (percentage of number of emission photons to the number of absorbed photons) of preferably at least 20%, more preferably at least 40%.
As the fluorescent dye, known dyes and pigments, such as dyes and pigments as described in xe2x80x9cDye Manualxe2x80x9d (pages 315-1109, compiled by Organic Synthesis Institute, 1970 and xe2x80x9cColoring Material Technology Handbookxe2x80x9d (pages 225-417, compiled by Coloring Material Institute, 1989) may be used. Particularly, dyes as described in xe2x80x9cLaser Dyesxe2x80x9d (Mitsuo Maeda, published by Academic Press, 1984) are preferred. Specifically, carbocyanine dyes in Table 4 at pages 26-29, phthalocyanine dyes in Table 11 at pages 74-75, xanthene dyes in Table 12 at pages 76-105, triarylmethane dyes in Table 13 at page 106, acridine dyes in Table 14 at pages 107-110, condensed ring compounds in Table 18 at pages 137-149, coumarin and azacoumarin dyes in Table 23 at pages 189-238, quinolone and azaquinolone dyes in Table 25 at pages 239-246, oxazole and benzoxazole compounds in Table 26 at pages 247-261, furan and benzofuran compounds in Table 29 at pages 273-275, pyrazoline compounds in Table 30 at page 276, phthalimido and naphthalimido compounds in Table 31 at page 277, peteridine compounds in Table 32 at page 282, and pyrylium, phosphorin, boraziadinium and pyridine compounds in Table 33 at page 283, may, for example, be mentioned. Further, diketopyrrolopyrrole compounds as disclosed in JP-A-58-210084, and perylene compounds as disclosed in JP-A-7-188178, may, for example, be used.
The fluorescent dye or the fluorescent pigment preferably has a maximum (peak) wavelength of the absorption spectrum within a range of from 350 to 500 nm, and the maximum wavelength of the fluorescence (emission) spectrum within a range of from 500 to 600 nm. More preferably, the maximum wavelength of the absorption spectrum is within a range of from 400 to 490 nm, and the maximum wavelength of the fluorescence spectrum is within a range of from 490 to 600 nm. Still more preferably, the maximum wavelength of the absorption spectrum is within a range of from 400 to 490 nm, and the maximum wavelength of the fluorescence spectrum is within a range of from 500 to 570 nm. Among the above-mentioned compounds, as examples of such a fluorescent dye or fluorescent pigment, carbocyanine dyes, xanthene dyes, triarylmethane dyes, acridine dyes, coumarin and azocoumarin dyes, phthalimido and naphthalimido compounds, pyrylium compounds, diketopyrrolopyrrole compounds and perylene compounds may, for example, be mentioned. Particularly preferably, the maximum wavelength of the fluorescence spectrum is within a range of from 500 to 555 nm. As examples of such a fluorescent dye or fluorescent pigment, carbocyanine dyes, triarylmethane dyes, coumarin dyes, phthalimido and naphthalimido compounds, diketopyrrolopyrrole compounds and perylene compounds may be mentioned.
Such a fluorescent dye or fluorescent pigment preferably has no (short) afterglow. Namely, one having a fluorescence life of at most 10xe2x88x922 second is preferably used. Further, it is preferred that such a fluorescent dye or fluorescent pigment is less likely to undergo aging or decomposition due to light or heat.
The phosphor to be used for each of the phosphor layers in the intensifying screen set of the present invention is not particularly limited in view of its composition, and at least one phosphor which emits lights having a high luminance when irradiated with X-rays, such as CaWO4, YTaO4, YTaO4:Tm, YTaO4:Nb, BaSO4:Pb, HfO2:Ti, HfP2O7:Cu, CdWO4, GdTaO4:Tb, Gd2O3. Ta2O5.B2O3:Tb, Gd2O2S:Tb, (Gd, Y)2O2S:Tb or Tm, may be used.
The binder resin to be used for the phosphor coating solution to produce the intensifying screen set of the present invention, is not particularly limited so long as it is a conventional binder for intensifying screens, such as nitrocellulose, cellulose acetate, ethylcellulose, polyvinylbutyral, linear polyester, polyvinyl acetate, a vinylidene chloride/vinyl chloride copolymer, a vinyl chloride/vinyl acetate copolymer, a polyalkyl-(meth)acrylate, polycarbonate, polyurethane, cellulose acetate butyrate, polyvinyl alcohol, gelatin, a polysaccharide such as dextrin, or gum arabic. The amount of the binder resin is particularly preferably from 2 to 10 wt % to the phosphors in the phosphor layers, not to decrease sharpness and durability of the obtained intensifying screen set.
The organic solvent to be used for preparation of the phosphor coating solution may, for example, be ethanol, methyl ethyl ether, butyl acetate, ethyl acetate, ethyl ether or xylene.
To the phosphor coating solution, a dispersing agent such as phthalic acid or stearic acid, or a plasticizer such as triphenyl phosphate or diethyl phthalate may be added, as the case requires.
As the supports to be used for the intensifying screen set of the present invention, a light-reflective support is used for the front intensifying screen, and a light-absorptive support is used for the rear intensifying screen.
In the intensifying screen set of the present invention, the light-reflective support is one having a reflectance at its surface of at least about 80% to lights having wavelength ranges of from 450 nm to 700 nm, and the light-absorptive support is one having a reflectance at its surface of at most about 5%.
Specifically, the materials to be the bases for the supports for the front and rear intensifying screens, may be one based on the same material for a support to be used for a conventional intensifying screen, such as a film of cellulose acetate, cellulose propionate, cellulose acetate butyrate, polyester such as polyethyleneterephthalate, polystyrene, polymethylmethacrylate, polyamide, polyimide, a vinyl chloride/vinyl acetate copolymer or polycarbonate, a baryta paper sheet, a resin-coated paper sheet, a normal paper sheet or an aluminum alloy foil. As the support for the front intensifying screen, one comprising the above-mentioned material as the base, and a light-reflective material such as titanium dioxide or calcium carbonate incorporated therein, or bubbles contained therein, may, for example, be used.
Further, instead of using the above-mentioned light-reflective support as the support, a light-reflective layer made of a reflective material having a high refractive index, and having a relatively little X-ray absorption, such as MgO, Al2O3, SiO2, ZnO, TiO2 or ZnS, as disclosed in JP-A-9-217899, may be formed between the support and the phosphor layer adjacent thereto. The light-reflective layer is preferably one having an excellent sharpness of reflection, and accordingly, it is preferably one having an adequately small layer thickness and presenting an adequately high reflectance. Accordingly, the light-reflective material to be used for the light-reflective layer is preferably a material having a small particle size with an average particle size of at most 0.5 xcexcm. The reflectance of the light-reflective layer is almost the same for the light-reflective support of the present invention, and the reflectance is preferably at least 85% in the case of preparing a reflective layer with a higher reflectiveness. In the case of forming the above-mentioned light-reflective layer between the phosphor layer and the support adjacent thereto also, a light-reflective support may be used as the support.
On the contrary, as the support for the rear intensifying screen, one comprising the above-mentioned material as the base and a light-absorptive material such as carbon black incorporated therein, may be used.
Further, instead of using the above-mentioned light-absorptive support as the support, a light-absorptive layer made of light-absorptive materials including carbon black, and having a reflectance at its surface of at most about 5%, preferably at most 3%, may preliminarily be formed between the support and the phosphor layer adjacent to the support. Also with respect to the rear intensifying screen, in the case of forming the above-mentioned light-absorptive layer between the phosphor layer and the support adjacent thereto, a light-absorptive support may be used as the support.
The reflectances of the support, the light-reflective layer and the light-absorptive layer to be the bases for the phosphor layer, are diffused reflectances at the surface. The reflectance can be obtained by using an integrating-sphere having a BaSO4 powder uniformly coated on the entire surface, for example, a 150 xcfx86 integrating-sphere (150-0901), with an autographic spectrophotometer Model U-3210 manufactured by Hitachi Ltd., and measuring the reflectance against the standard white board (210-0740). The wavelength for measuring the reflectance is determined by considering the emission wavelength of the phosphor constituting each intensifying screen. When the emitted lights have plural wavelengths peaks, or when the peak is broad, measurements are carried out at the respective wavelengths, and the reflectance is obtained by weighted average by considering the intensity of emitted lights.
As mentioned above, by employing a light-reflective support as the support for the front intensifying screen, or forming a light-reflective layer between the phosphor layer and the support, and by employing a light-absorptive support as the support for the rear intensifying screen, or by forming a light-absorptive layer between the phosphor layer and the support, and by forming a plurality of phosphor layers thereon to obtain a multi-layer structure, the light-absorptive support can be used for the rear intensifying screen at the sensitivity range which has conventionally be obtained only by using the light-reflective support for the rear intensifying screen. Further, by using a light-reflective support for the front intensifying screen to decrease the weight of the phosphor coated on the front side, in order to effectively utilize the emitted lights from the rear intensifying screen, an intensifying screen set having a higher sensitivity and a higher sharpness, as compared with the conventional intensifying screen set, can be obtained. Further, by employing a light-absorptive support for the rear intensifying screen, the weight of the phosphor coated tends to increase as compared with the case where the light-reflective support is employed for the rear intensifying screen, at the same sensitivity, whereby granularity will improve.
Further, on the surface of the formed phosphor layer in the intensifying screen set of the present invention, a protective layer may be provided as the case requires. The protective layer is formed in such a manner that a cellulose derivative such as cellulose acetate, nitrocellulose or cellulose acetate butyrate, or a resin such as polyvinyl chloride, polyvinyl acetate, a vinyl chloride/vinyl acetate copolymer, polycarbonate, polyvinylbutyral, polymethylmethacrylate, polyvinylformal or polyurethane, is dissolved in a solvent to prepare a protective layer coating solution having a suitable viscosity, which is then coated on the uppermost phosphor layer (the side from which the emitted lights are taken out to the exterior) preliminarily formed on the support, followed by drying; or a preliminarily formed protective layer such as a transparent film of e.g. polyethylene terephthalate, polyethylene naphthalate, polyethylene, polyvinylidene chloride or polyamide, is laminated on the phosphor layer.
In the case where a protective layer is provided for the intensifying screen set of the present invention, the protective layer is preferably a light-scattering protective layer having light-scattering characteristics imparted thereto by e.g. uniformly dispersing light-scattering substance such as titanium dioxide or calcium carbonate in a form of fine particles, and having a transmission of at least 40% and a haze ratio of at least 20%, since light components having a large angle of incidence from the phosphor layer to the protective layer, which will lower the sharpness, can be effectively removed, whereby sharpness will improve.
The intensifying screen of the present invention can be produced by another method than the above-described production method. For example, a protective layer is preliminarily formed on a smooth substrate, a plurality of phosphor layers are successively formed thereon, and the phosphor layers are separated from said substrate together with the protective layer, and a support is bonded to the phosphor layer (to the other side to the protective layer), to produce an intensifying screen.